zero plus dialing inc / blocking calls

We have a son incarcerated at Vienna Illinois. We have him calling collect from time to time through this company. Now they say that they can block his calls because our daughter who lived in Iowa had a phone bill years ago with Qwest that wasn't paid. That was when she was married. She lived with us for a while a year ago and her name was on our Frontier account but it was also in our name. We have always paid our phone bills and cannot understand how they can do this when it is not our account that was never paid.